The present invention is concerned with the field of burner technology. It relates to a burner for the production of hot gases according to the preamble of claim 1.
The fluid-dynamic stability of a gas turbine burner is of critical importance for the occurrence of thermoacoustic oscillations. Fluid dynamic instability waves which arise at the burner lead to the formation of vortices (coherent structures) which affect the combustion and can lead to the periodic release of heat with the pressure fluctuations (thermoacoustic oscillations) connected therewith. Thermoacoustic oscillations represent a danger for any kind of combustion application. They lead to high amplitude pressure oscillations and to a limitation of the operating range, and can increase pollutant emissions. This particularly applies to combustion systems with little acoustic damping. In order to make high power conversion possible over a wide operating range in relation to pulsations and emissions, an active control of the combustion oscillations can be necessary.
Numerous proposals have already been made in the past, and possibilities shown, as to how the undesired thermoacoustic oscillations can be damped or completely suppressed in such burners, particularly in so-called double cone burners such as described, for example in EP-A2-0 881 432.
It is proposed in EP-A1-0 918 152 to control the thermoacoustic oscillation in a combustion system by arranging in the region of the burner, means for acoustic excitation of the working gas. This is of course connected with an additional apparatus and control expense. A similar acoustic design (EP-A1-1 050 713) works with an active suppression by means of a feedback control loop with a corresponding phase rotation.
In EP-A1-0 987 495, it is proposed to admix an inert gas, e.g., N2, CO2, or the like additionally to the fuel flow, in order to minimize thermoacoustic oscillations in gas turbine combustion chambers. However, this means an additional supply and duct system for the admixed inert gas.
Other solutions modify the geometry of the burner, particularly at the burner outlet: it is proposed in EP-A1-1 002 992 to arrange numerous nozzles in the burner along the periphery on the inside of the burner outlet, which introduce axial vortex intensities into the flow by the injection of air at an angle to the flow direction, for the control of flow instabilities in the burner.
EP-A1-0 985 877 furthermore proposes to accelerate the flow in the axial direction for minimizing thermoacoustic oscillations in gas turbine combustion chambers, in that the burner outlet is made nozzle-like, or additions of nozzle-like shape are installed on the burner.
Finally, in EP-A1-1 048 898 a burner (double cone burner) is disclosed in which numerous additions projecting into the flow are provided in order to introduce axial vortex intensities in the burner cone.
The present invention, on the contrary, proceeds from the following considerations: coherent structures play a critical role in mixing processes between air and fuel. The dynamics of these structures consequently affect the combustion and thus the liberation of heat. Control of combustion instabilities is possible by acting on the shear layer between the fresh gas mixture and the recirculated exhaust gas (see, e.g., Paschereit et al., xe2x80x9cStructure and Control of Thermoacoustic Instabilities in a Gas Turbine Burner Combustionxe2x80x9d, Science and Technology, Vol. 138, pp. 213-232 (1998)). In particular, this offers the formation of coherent structures by means of having effects on the outflow boundary layer at the burner outlet.
The invention has as its object to provide a burner in which thermoacoustic oscillations can be limited or completely suppressed by very simple constructional means.
The object is attained by means of the totality of the features of claim 1. The basic concept of the invention is to affect the formation of coherent structures in a manner such that the occurrence of high-frequency, combustion-driven oscillations is prevented. Coherent structures are to be understood here as flow vortices, which arise due to flow instabilities in the shear layers which form at the burner outlet. The effect of coherent structures on the combustion instabilities is at its most pronounced when the flow instability has crossed its highest growth rate and the vortices have reached their maximum size. The axial position of the highest growth rate can be affected by, among other things, changing the thickness of the outflow boundary layer. By the prevention of the occurrence of vortex structures in the region of the flame, a periodic release of heat is prevented. A periodic release of heat, however, would be the basis for the occurrence of thermoacoustic oscillations, which are thus prevented.
A preferred embodiment of the invention is thus characterized in that the means for changing the thickness of the outflow boundary layer include a shear layer fence which runs along the outlet edge of the burner outlet and projects into the combustion chamber with its height substantially parallel to the flow direction. By means of the shear layer fence, which preferably has a height of a few millimeters, the thickness of the outflow boundary layer is increased in a particularly simple manner, and thus the vortex formation is displaced in the axial direction out of the region of the flame, with the cessation of periodic releases of heat which are connected with the vortex formation.
The burner is preferably constituted as a double-cone burner, and includes at least two hollow, conical partial members which are nested one in the other in the flow direction and whose mid-axes run mutually offset, such that adjacent walls of the partial members form tangential air inlet channels for the inflow of combustion air into the internal space bounded by the partial members, with the edges of the partial members toward the combustion chamber forming the outlet edges of the burner outlet.